Internet Explorer hit by zero-day exploit, temporary fix issued

Microsoft is urging users of Internet Explorer to download a free security tool, enhanced Mitigation Experience Toolkit (EMET), as an interim measure against a previously unknown zero-day exploit in its web browser software that is under active malware attack by hackers.
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SO youre telling us that yet again MS has released a product that is flawed. How does this happen? I mean dont they quality test their own product? Or hand it over to some paid hackers and let them tear into it? That seems the least they could do. If some enterprising hacker can find this, why didnt the ppl who made it?